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View Full Version : The Hype of ac-3 and others?


datauser
16th September 2011, 11:04
I think that mp3 has debunked the myth of ac-3 being the best out there and now aac is certainly heading especially Nero to par excellence.

Looking into it from reading on the Net, mp3 still has its uses and aac at 160kbp/s is only slightly marginally better at mp3 160kbp/s in terms of efficiency, but aac at lower bitrates blows mp3's socks off so they say in subjective hearing tests.

Now ac-3 is very good quality no doubt, but are these 2.0 stereo surround tracks any good? I always get confused between the differences between 2.0 surround, or stereo surround. Something to do with the matrix which unfolds in 4 discrete channels using a suitable decoder they tell me. Any way I converted a 5.1 track from an action based movie to mp3 160kbp/s and compared it with the untouched 192kbp/s Dolby stereo surround 2.0 track and the mp3 was vastly superior. People just like to see the Dolby sign on their players and are a bit duped by it still.

The background noise was retained in the mp3 track 2.0 track eg bird singing, cars driving past quickly(not bad at all for a 2.0 surround mp3 downmix) etc, sounds which were clearly retained from the 5.1 mix, but when I played the original Dolby 2.0 stereo surround track through a certified Dolby decoder to compare these noises, the intensity of sound was just not there nor the same background info, obviously as it is not the 5.1 mix, but still very disappointing as I expected some spectral field of unfolding background surround noise. I assumed the extra matrix channels would kick in with the matrixed info at least a little better as Dolby so clearly hypes its prologic algorithmns up? Good old Lame parameters do retain a lot of info at 160kbp/s from 5.1 tracks and appears superior to professionally encoded Dolby 2.0 tracks from same source. I converted the 5.1 to 128kbp/s and it sounded no better than the Dolby original 2.0 track, in fact about the same.

Verdict for me anyway. Mp3 is more efficient than ac-3 at lower bitrates and the cut off point is definitely 160kb/s either joint stereo or normal stereo for good reproduction of info from a 5.1 track downmixed to 2 channels. Just don't expect too much from Dolby 2.0 tracks when comparing it with 5.1 mixes of same source!

What are your experiences?

hello_hello
16th September 2011, 11:56
Verdict for me anyway. Mp3 is more efficient than ac-3 at lower bitrates and the cut off point is definetly 160kb/s either joint stereo or normal stereo for good reproduction of info from a 5.1 track downmixed to 2 channels.

I don't think anyone would argue AC3 requires higher bitrates than MP3 for the same quality.
Depending on whether my encoding program is converting the audio to MP3 (AutoGK if I'm converting to AVI) or whether I do it manually, the audio is either mixed down to two tracks for surround sound decoding, or simply mixed to ordinary stereo. Either way I just use a bitrate of 128 for MP3 as I'm not interested in Dolby stereo/surround decoding so I'm not fussed as to how much of the surround information is lost. I can't say I've ever tried comparisons at different bitrates, but I'm not sure anyone would argue with your verdict.

Personally I think if you're serious about multichannel audio, it's better to keep the original AC3 multichannel audio if available, or depending on the format, convert the DTS multichannel audio to multichannel AAC etc.

tebasuna51
16th September 2011, 12:27
...
Verdict for me anyway. Mp3 is more efficient than ac-3 at lower bitrates and the cut off point is definitely 160kb/s either joint stereo or normal stereo for good reproduction of info from a 5.1 track downmixed to 2 channels. Just don't expect too much from Dolby 2.0 tracks when comparing it with 5.1 mixes of same source! ...

I agree. Talking about codec efficiency AAC > MP3 > AC3

datauser
16th September 2011, 12:37
You can't beat a good 5.1 surround system, and every 2.1 virtual TV I tested is paltry by comparison. Surround sound for is not an issue too, stereo is more than enough, but during my subjective tests on numerous tracks I was quite surprised to see how much surround info is lost from 160 mp3 to 128 bitrate from 5.1 tracks even though the option to downmix to surround is defaulted in many encoders at 128kbp/s.

I read that many TV's now are using a subwoofer/bass redirection to enhance the 2.1 systems better, but is probably hype too and another thread.;)

SeeMoreDigital
16th September 2011, 12:54
Given that the Blu-ray disc format offers us the ability to get our hands on some good quality "lossless" surround sound sources. It's now possible (for those interested) to generate their own "lossy" surround sound comparison tests.

Personally, I quite like 5.1Ch AAC-LC at 256Kbps. It sounds pretty good when compared to 5.1Ch AC3 at 448Kbps.

That said, in order to make a fair comparison, you really need a playback device that can send "discrete" surround sound audio via HDMI to a decent amplifier ;)

datauser
16th September 2011, 14:36
That said, in order to make a fair comparison, you really need a playback device that can send "discrete" surround sound audio via HDMI to a decent amplifier ;)

I play some of my 5.1 mp4 stuff through my TV's usb port and downmixed via the TV HDMI to stereo 2.0. My only quibble with aac is that it can sometimes 'lose the vocals strength,' whether this is due to the normalizing process used to make the mp4's audio, or decoding device used I don't know for sure? But this equally applies to 2.0 aac as well, I find in my tests. Heavy bass action which drowns out the voices so you have to be constantly turning it up then back down for the action. That said I have noticed this with the straight playback of many blu-ray discs as well, unlike DVD playback or avi container/mp3 tracks or test tracks I've done with Nero format. Time to invest into better playback equipment may be the answer for me!

hello_hello
17th September 2011, 18:43
I guess in a perfect world if you convert lossless or high quality 5.1ch audio to 5.1ch AAC you shouldn't be able to hear the difference.... as long as you use a high enough quality setting I suppose.
How are you converting and/or mixing down to AAC and what sort of quality setting do you use when you're "losing the vocals strength"? I can't say it's something I've ever noticed. Maybe it's just the way the original audio track is. Personally I think movie audio tends to be a somewhat overly dynamic, but converting to AAC shouldn't make it worse.

datauser
19th September 2011, 17:16
Maybe it's just the way the original audio track is. Personally I think movie audio tends to be a somewhat overly dynamic, but converting to AAC shouldn't make it worse.

Probably that, or to do with dynamic compression.

hello_hello
19th September 2011, 17:42
Probably that, or to do with dynamic compression.

Well unless you enable the dynamic compression when decoding it shouldn't have any effect on the encoded AAC output.